Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02794356 2012-10-31
TREATMENT OF PRODUCED WATER WITH SEEDED EVAPORATOR
FIELD
[0001] This specification relates to a method and apparatus for treating
water, for
example produced water.
BACKGROUND
[0002] Canadian Patent 2 509 309 by Heins, entitled Water Treatment
Method for
Heavy Oil Production Using Calcium Sulfate Seed Slurry Evaporation, describes
an
evaporator based method of treating produced water from heavy oil production.
Produced
water is first treated in a de-oiling step to reduce oil and grease to about
20 parts per million
or less. De-oiled water is introduced into an evaporator to produce a
distillate and an
evaporator blowdown containing concentrated residual solids. The distillate is
used,
optionally after further polishing, to create steam in a once-through steam
generator or other
boiler. The evaporator blowdown may be further treated in a crystallizer.
INTRODUCTION
[0003] The following summary is intended to introduce the reader to
the detailed
description to follow and not to limit or define the claimed invention.
[0004] Evaporator blowdown may require treatment to remove silica
and/or other
contaminants before it can be disposed or re-used. A crystallizer, for
example, ordinarily
treats evaporator blowdown to recover water and produce dry, or easily dried,
crystals made
up of solids precipitated from the blowdown. However, when a crystallizer is
used to treat
produced water, particularly produced water from heavy oil production, the
crystallizer may
instead produce a slurry. The slurry contains salts, for example silica salts,
at concentrations
above their ordinary limits of solubility in water and yet the solids are not
crystallized and
cannot be easily separated from the slurry. Concentrated organic compounds
from the
produced water remaining in the slurry may interfere with crystal growth and
precipitation
even for over saturated salts in the slurry.
[0005] In a method and apparatus described in this specification, a
source of ions, for
example magnesium or calcium ions, is added to the evaporator. A solvent, for
example
diisopropylamine (DIPA), isopropylamine (IPA), methanol or alcohol, is added
to the
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blowdown. The solvent is later recovered from the solvent-blowdown mixture.
Solids are
separated from the blowdown before or after the solvent is recovered.
Optionally, the
blowdown may be further concentrated, for example in a crystallizer, before
the solvent is
added. Optionally, treated blowdown may be returned to the evaporator.
[0006] The method and apparatus are useful, for example, in providing an
alternative
means of treating produced water, or other water containing dissolved salts
and organic
compounds. The method and apparatus may be incorporated into a method and
apparatus
for extracting heavy oil.
BRIEF DESCRIPTION OF THE FIGURES
[0007] Figure 1 is a schematic process flow diagram of a produced
water treatment
system combined with a heavy oil extraction system.
DETAILED DESCRIPTION
[0008] Figure 1 shows a water treatment system 10 in combination with a
heavy oil,
for example bitumen, recovery system 12. The heavy oil recovery system 12 has
a boiler 14,
an injection well 16, a production well 18 and an oil-water separator 20. The
boiler 14 may
be, for example, a once through steam generator (OTSG) or a packaged boiler.
The boiler
14 produces steam 22 which flows to the injection well 16. Injection well 16
carries the
steam 22 into a geologic formation 24 containing heavy oil, for example
bitumen in the oil
sands areas of Alberta, Canada. The steam 22 fluidizes bitumen in the
formation 24 and
later condenses. An oil and water mixture 26 is produced and withdrawn from
the
production well 18. The oil and water mixture 26 is sent to the oil-water
separator 20.
Product oil 28 is removed from the oil-water separator 20 for further
refining. Produced water
30, which remains after the product oil 28 is removed from the oil and water
mixture 26, flows
to the water treatment system 10 for further treatment. The heavy oil recovery
system 12
may be, for example, a steam assisted gravity drainage (SAGD) or cyclic steam
stimulation
(CSS) system.
[0009] The water treatment system 10 has a de-oiling unit 50, an
evaporator 52,
optionally a crystallizer 54, and a blowdown treatment system 56. The de-
oiling unit 50
receives the produced water 30 and removes additional oil 51, for example
emulsified oil,
from the produced water 30 to produce de-oiled water 58. The de-oiling unit 50
may be, for
example, a dissolved gas flotation unit, a walnut shell filtration unit, a de-
oiling polymer
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mixing tank, a sorbent unit, a membrane filtration unit or another device. A
suitable
evaporator 52 and crystallizer 54 are available from GE Ionics, Inc.
[0010] De-oiled water 58 is treated further in the evaporator 52.
Optionally, the de-
oiled water 58 may pass through intermediate treatment steps on the way to
evaporator 52.
However, a warm lime softener is not required and preferably not provided.
[0011] Evaporator 52 may be, for example, a mechanical vapour
recompression
(MVC or MVR) evaporator. The evaporator 52 may use a falling film or other
internal
arrangement. The evaporator 52 may have associated equipment used, for
example, to alter
the pH of the de-oiled water or remove gasses or alkalinity from the de-oiled
water 58. The
evaporator 52 removes water vapour from the de-oiled water 58. The removed
water
vapour, when condensed, forms distillate 60 which is returned to the boiler 14
for re-use in
the heavy oil recovery system 12, optionally after further treatment.
[0012] The evaporator 52 also has a brine recirculation loop 62. A
seeding chemical
63 is added to the evaporator 52, for example by injection into the brine
recirculation loop 62
as shown, the sump of the evaporator, a feed line to the evaporator or by
other means. The
seeding chemical 63 may be, for example, a magnesium oxide or calcium salt.
The seeding
chemical may be added in a dry form or in solution. A suitable magnesium salt
is
magnesium oxide. A suitable calcium salt is calcium sulfate (gypsum).
[0013] An evaporator blowdown 64 is withdrawn from the brine
recirculation loop 62
and fed to the blowdown treatment system 56. Optionally, the blowdown
treatment system
56 may be located within the brine recirculation loop 62. The evaporator
blowdown 64, and
the water in the brine circulation loop 62, have a high concentration of
various contaminants
that were present in the de-oiled water 58. In particular, the evaporator
blowdown 64 and
water in the brine circulation loop 62 have a high concentration of silica.
The evaporator
blowdown 64 is therefore not suitable for discharge in many locations. The use
of a seeding
chemical and a solvent (to be described below) may be particularly useful when
the
evaporator blowdown 64 or liquid in the brine recirculation loop 62 has a
water soluble
organics concentration (WSO) of 3 wt% or more and the WS0 concentration is 20%
or more
of the total solids (TS) concentration. TS is measured by heating a sample to
105 degrees C
to remove water, then dividing the dry sample mass by the original sample
mass. Total
dissolved solids (TDS) is is measured by heating a sample to 550 degrees C to
remove
water and organics, then dividing the dry sample mass by the original sample
mass. WSO is
calculated as the difference between TS and TDS.
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[0014] In the water treatment system 10, the evaporator blowdown 64
may be treated
further in the optional crystallizer 54. Crystallizer 54 removes additional
water vapour from
the evaporator blowdown 64 which, when condensed, forms second distillate 66.
Second
distillate 66 may be returned to the boiler 14 for re-use in the heavy oil
recovery system 12,
optionally after further treatment.
[0015] The crystallizer 54 also produces a crystallizer blowdown 68.
Crystallizer
blowdown 68 has an even higher concentration of silica and other dissolved
solids compared
to the evaporator blowdown 64. Under some conditions, particularly when
treating produced
water 30 from a steam or water flood assisted oil recovery operation, the
crystallizer 54
produces a crystallizer blowdown 68 in the form of a slurry having a high
solids
concentration. Silica and other dissolved solids are present in the slurry
under super-
saturated conditions but do not precipitate and settle from the slurry. For
example, the
crystallizer blowdown 68 may have a total solids (TS) content of 25 wt% or
more or 50 wt%
or more.
[0016] The evaporator 52 and crystallizer 54 are types of thermal treatment
devices,
alternatively called distillation devices. Optionally, alternative thermal
treatment devices or
concentrating devices, in one or more stages, may be used to create slurry
similar to the
crystallizer blowdown 68 or mixtures similar to evaporator blowdown 64 or
water in the brine
recirculation loop 62. Total organic carbon (TOC) in any of these mixtures 62,
64, 68 may be
2 wt% of more; total dissolved solids (TDS) may be 10 wt% or more; and, silica
may be 1.5
wt% or more. Blowdown treatment system 56 may be located to receive
crystallizer
blowdown 68 as shown, to receive evaporator blowdown 64 without there being a
crystallizer
54, or to be part of brine recirculation loop 62 in which case evaporator
blowdown 64 stream
may be omitted, or retained as a smaller system bleed with or without
evaporator 54.
[0017] The blowdown treatment system 56 shown comprises a reaction vessel
70, a
solvent dosing system 72 and a solvent recovery system 74. The crystallizer
blowdown 68
(or evaporator blowdown 64 or recirculating brine 62) and solvent from the
solvent dosing
system 72 are added to the reaction vessel 70 and mixed together. The reaction
vessel 70
may be, for example, a mixing tank or a conduit with in-line mixing. The
solvent may pass
through a heat exchanger 88 to solvent and thereby cool the crystallizer
blowdown 68 (or
evaporator blowdown 64 or recirculating brine 62) before it enters the
reaction vessel 70 if
required to maintain a temperature in the reaction vessel 70 below the boiling
point of the
solvent or to maintain a temperature at which the solvent and water are
miscible.
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Precipitated solids 76 form, settle and are removed from the bottom of the
reaction vessel
70, which acts as an integrated solid-liquid separation unit. Alternatively,
the precipitated
solids 76 may be removed by a separate solid-liquid separation unit such as a
centrifuge,
filter press, hydrocyclone, filter or clarifier. In a further alternative,
solvent may be recovered
from the crystallizer blowdown 68 (or evaporator blowdown 64 or recirculating
brine 62)
before or after the precipitated solids 76 are removed.
[0018] The solvent is added in a mass ratio of at least 1:1, or about
5:1 or more,
solvent to crystallizer blowdown 68 (or evaporator blowdown 64 or
recirculating brine 62).
[0019] The remaining blowdown-solvent mixture 78 is sent to the
solvent recovery
system 74. The solvent recovery system 74 may comprise a heated decanting
vessel. In
the case of DIPA, heating the blowdown-solvent mixture 78 to about 74 degrees
C causes
the solvent to separate from the water and form a liquid solvent phase over
the water. The
solvent is decanted and drawn off from the top of the solvent recovery system
74.
Alternatively, other solvents such as IPA, methanol or ethanol can be
extracted from the
blowdown-solvent mixture 78 by distillation. Other solvent extraction means
may be
appropriate for other solvents. Recovered solvent 80 is sent to the solvent
dosing system 72
for re-use in the blowdown treatment system 56.
[0020] The precipitated solids 76 may include some organic compounds.
The
precipitated solids 76 may be treated to remove the organic compounds if
required to meet
discharge or disposal requirements relating to the precipitated solids. For
example, the
precipitated solids 76 may be dried, heated, exposed to UV radiation, exposed
to
microorganisms, embodied in cement, or washed with water, the solvent used in
the solvent
recovery system 74, or another solvent.
[0021] A waste brine 82 is also produced. In a case where the
blowdown treatment
system 56 is located within the brine recirculation loop 62, the waste brine
82 flows to the
evaporator 52 to complete the brine recirculation loop 62. In other options,
the waste brine
82 may be discharged or recycled to the water treatment system 10, for example
by being
sent back to the evaporator 52. Treatment unit 90 may extract remaining
solvent or remove
organic contaminants form the waste brine 82. Optionally, chemicals may be
added to the
waste brine 82 to oxidize remaining organic compounds. Alternatively, or
additionally, the
organic compounds may be digested biologically. In a further alternative, the
organic
compounds may be recovered for their value as industrial chemicals. The
treated waste
brine 82 may be returned to the water treatment system 10. However, it may be
preferable
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in some cases to remove the waste brine 82 from the water treatment system 10,
optionally
after treating the waste brine 82 if required to meet any standards required
for discharge or
disposal. In this way, the waste brine 82 provides a bleed of water soluble
organic
compounds from the water treatment system 10 to inhibit the accumulation of
any organic
compounds that are not effectively removed by any process in the water
treatment system
10.
[0022] This written description uses examples to disclose the
invention and also to
enable any person skilled in the art to practice the invention. The patentable
scope of the
invention is defined by the claims, and may include other examples that occur
to those skilled
in the art.
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